Pharmacological agents are being developed to modulate phosphotyrosyl (pTyr)-dependent cell signalling. Emphasis is on inhibitors of pTyr- dependent binding interactions which are mediated by "src homology 2" (SH2) domains or "phosphotyrosine-interaction domains" (PIDs) and on protein-tyrosine phosphatase (PTP) inhibitors. Central to both of these efforts is the development of new pTyr mimetics which afford either increased stability toward enzymatic degradation by PTPs, increased cellular bioavailability or increased affinity. Among the pTyr mimetics developed are phosphatase-resistant fluorinated phosphonate analogues (for example, F2Pmp) which when incorporated into peptides retain high affinity for SH2 domains, PIDs and PTPs. New non-phosphorus-containing pTyr mimetics have been developed (OMT and its fluorinated analogue FOMT) which potentially afford new avenues of prodrug delivery. Conformationally constrained pTyr analogues which mimic the X-ray derived structures of pTyr residues bound to SH2 domains have been prepared as high affinity pTyr mimetics. Additionally, heteroaryl-containing pTyr mimetics have been prepared using our recently developed chiral synthesis of the differentiating antibiotic azatyrosine, which are designed to afford higher affinity and specificity for SH2 domain binding interactions. New PTP inhibitors have been designed and synthesized based on the X-ray structure of our lead small molecule inhibitor bound to the PTP 1B enzyme. A 100% increase in affinity has resulted from the first analogue developed by this effort. Peptidomimetics have also been designed and are being prepared based on the NMR solution conformation of our cyclic SH2 domain inhibitory peptide. Discoveries from each of these distinct areas of investigation will be combined in the design and development of second generation inhibitors directed against pTyr- dependent signalling.